Characterization of doped BST thin films for microwave tunable device applications

Abstract

The prospects of using ferroelectric thin films for the fabrication and development of frequency and phase agile microwave devices have increased in past few years due to improvements in ferroelectric thin film processing techniques as well as innovative circuit design. In microwave devices, ferroelectric thin film is used as tuning layer through the nonlinear electric field dependence of the relative dielectric constant. In this study, we report on an investigation of the doping effects of several dopants in $Ba_{0.6}Sr_{0.4}TiO_3$ as a significant factor affecting the dielectric properties and discuss the dependence of tunability and dielectric loss on the characteristics of dopants at low frequency(100kHz). Firstly, to examine the effect of LSCO buffer layer on BST tunability, two types of capacitors such as Pt/BST/Pt and Pt/BST/LSCO/Pt were compared. Compared with the crystallinity of BST film on Pt electrode, BST film grown on LSCO layer showed an improved (110) phase formation. LSCO buffer layer can promote the growth of BST films due to the structural match between BST and LSCO. At the applied voltage of 5V, the BST film deposited on LSCO/Pt shows a tunability of 62.8%. It is noted that the tunability of the BST film grown on LSCO buffer layer is considerably higher than that (12.6%) obtained at the same deposition condition by using Pt electrode. Secondly, in this work, Ni doped BST/LSCO/Pt films were fabricated to investigate the acceptor doping effect on dielectric properties of BST films, such as tunability and dielectric loss. Both dielectric loss and tunability were found to decrease as a function of increasing Ni concentration. This decrease in dielectric tuning with addition of acceptor dopants is not surprising and has been reported in other studies for doped BST thin films. (16,37) The decreased dielectric loss of Ni doped BST/LSCO/Pt film is attributed to suppression of the concentration of oxygen vacancies and enlagement of grain bounda...